1. Field of the Invention
The present invention relates generally to the field of optical output devices, and more specifically to a method and apparatus for eliminating curvature in the scan of an optical output device.
2. Related Art
Although applicable to a wide variety of optical output devices, the present invention finds particular utility in a Raster Output Scanning (ROS) apparatus. ROS has become the predominant method for imparting modulated light information onto the photoreceptor in printing apparatus used, for example, in digital printing, and has found some application in other image forming operations such as writing to a display, to photographic film, etc. Consider, for illustration purposes, what is perhaps the most common application of ROS, digital printing. As is known, the scanning aspect thereof is conventionally carried out by a moving reflective surface, which is typically a multifaceted polygon with one or more facets being mirrors. The polygon is rotated about an axis while an intensity-modulated light beam, typically laser light, is brought to bear on the rotating polygon at a predetermined angle. The light beam is reflected by a facet and thereafter focussed to a "spot" on a photosensitive recording medium. The rotation of the polygon causes the spot to scan linearly across the photosensitive medium in a fast scan (i.e., line scan) direction. Meanwhile, the photosensitive medium is advanced relatively more slowly than the rate of the fast scan in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. (Although, for the purposes of example, this discussion is in terms of ROE; apparatus, as will become apparent from the following discussion, there exist many other scanning and non-scanning system embodiments of the present invention. However, as a convention, the word "scan" when referring to fast and slow scan directions will be used with the understanding that actual scanning of the spot is not absolutely required.) Data in each of the fast and slow scan directions is generally sampled. The sampling rate of the slow scan direction data equates to 300 lines per inch or more in many printing apparatus. The light beam is intensity-modulated in accordance with a serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as sheet paper.
An important characteristic of the multiple scan lines that comprise the latent image is the relative spacing between different pairs of lines at each position in the fast scan direction. High quality printing requires scan lines that are accurately positioned relative to each other in the slow scan direction and accurately linear over the entire scan width. U.S. Pat. Nos. 5,208,456, 5,204,523, and 5,212,381, and U.S. Patent application Ser. No. 07/747,039, the disclosures of which are incorporated herein by reference, disclose methods and apparatus for accurately positioning the scan lines at the beginning of each scan but do not teach maintaining linearity over the entire scan width.
Scan line linearity is a significant problem in scanners employing multiple beams, such as disclosed in U.S. Pat. No. 4,404,571, because curvature , also called "bow", occurs in the output scan line when the optical source is displaced from the optical axis of the lens system. This bow can be eliminated in the single laser ROS by accurately aligining the optical source on the optical axis. Because it is impossible to align more than one source on the optical axis, however, with a multiple beam source it is impossible to eliminate bow simultaneously in all scanned lines. Furthermore, as shown in FIG. 1, the direction of scan line curvature on one side of the optical axis is opposite to the direction of scan line curvature on the other side of the optical axis. Consequently the relative spacing between simultaneously scanned lines changes across the scan line in a multiple beam scanner.
Differential bow in the slow scan direction is a significant problem in multiple beam scanners. Deviations in the relative spacing between scan lines of less than 1% of the nominal line spacing may be perceived in a halftone, continuous tone, or color image. Such a stringent requirement implies a need for a position control in the slow scan direction during each line scan in order to compensate for the inherent differential bow.
Known position control schemes such as rotating mirrors and translating roof mirrors are too large to move precisely and quickly in order to compensate for deviations of the spot position during a scan. Therefore, there is presently a need in the art for spot position control apparatus and methods which provide improved continuous, fast, very high resolution deflection of an optical beam in the slow scan direction during each scan to compensate for scan line curvature in output scanners.